heater trouble

[Rollie396]

I'm having trouble with the heater in my 80 (no ac).A bit of history first.The heater has always blown super hot since I got it.The only issue I had is the hot/cold lever didn't function,so it was either full bore or nothing.
I went on a 3hr roadtrip the other day and it was cool outside (-25c) but the heater put out great.I had it on with the fan on low and the window cracked 1/2".It was comfortable.When I got to the farm I pulled the truck in and did a bit of maintenance.I retorqued the heads which were pretty good,the outside row took a few pounds but nothing serious.I played with choke on the 1406 a bit more (a thread on it's own ),and then I decided to fix the hot/cold lever.
Turned out that a po discoed the cable and tied the door in the full hot position.I hooked the cable up again and tested it,everything seemed fine so I buttoned it up,gave myself an attaboy and moved on.
So today I head for home and it's about the same temp as the day I drove up that way.I noticed the heat wasn't anywhere near as hot as before and it seemed to get cooler the faster I drove and the longer I was on the road.To add to that I noticed the water temp rising a little,so I figured I was running low on coolant somehow.I pulled over,shut the truck down and started checking hoses.The lower rad hose was warmish,both heater hoses warm/hot and upper rad hose hot.I carefully removed the rad cap to check the level and lost a bit doing so but it wasn't much.I topped it up and refired the truck with the cap off.The level dropped immediately so I added another 1/2gallon or so to top it up and put the cap back on.Just for my own piece of mind,I checked the oil it was clean and at the proper level,so I continued on.The engine ran at normal temp the rest of the way,but still no real heat.The only thing that really changed here was I "fixed" the hot cold flapper.What could I have messed up in doing so?
Sorry for the novel.Thanks for any help.

[old Rusty C10]

its the typical cable operated flapper? prolly the cable came off or something. Im betting there was another issue which is why it is tied up open...

[Rollie396]

Sorry,yes it is cable operated.Once I hooked the cable back up,it seemed to function fine.I flipped it all the way to the hot side and pulled the cable off to see if the door was all the way closed.It was.I did the same on the cold side and it was slightly open still,but it was very little.I can still hear/feel the flapper moving so I'm pretty sure the cable is still on.I'm thinking you're right about another issue,just curious about what it could be?

[old Rusty C10]

im thinking it may not open all the way with the cable but enough you get some heat.. its also possible that the cable shifts so the arm goes all the way over in the selector but the door itself doesnt move enough

[MR .T]

Mine sucks too. I checked the cable and it seems to operate fine, I think my heater core is partally plugged.

[Rollie396]

I think the fact that the problem started after I hooked up the door again is no coincidence.Is there some kind of seal between the flapper and the cold side to prevent cold air from entering when it's in the full heat position?I'm wondering if there is a seal,did it fall away when I moved the door and allow cold air in?It does get worse at highway speed.I did test the door by moving the control panel lever all the way to the hot side then removed the cable from the heater box and checked to be sure the flapper wouldn't close any further.It wouldn't.
I'll be digging into it further next days off,but any advice between now and then will give me more to consider.I did some searching around on the boards and have seen similar situations,but most are to do with ac systems and vacuum valves?Mine doesn't have that.

[old Rusty C10]

heres some more pics of the heater only box removed from the truck the lower cable goes to the side of the heater box and the other one is for the location of where the air comes out

[bumslie75]

sorry not tryin to jack this thread but i cant move my temp control its on cold and thats all i mean no amount of force can move it why would that be

[oldschool18]

if you were saying you were low on fluid you thermostat could have stuck and then it might of pumped fluid out, get it hot and check your heater core hoses and see if they both get hot or not........if its still not getting warm and you keep losing fluid see if it gets wet at the bottle if it is I would try new thermostat......had same prob with my 77 monte carlo with a 305

[old Rusty C10]

bumslie the narrow end of the heater box is the side that the hot and cold switch moves. shown in picture 2. I would pop the cable off the door there and see if you can move the switch. If you can then the cable and your switch are good and the door is stuck. It you cant then re attach it pull the heater control and unattach it there and check it. If the cable moves its the switch if it doesnt then its the cable which are known to freeze up and rust at times.

[bumslie75]

cool thanks guys i will try that tonight

[TDS-MN]

On my '85 w/o a/c, I have to disconnect the cable and set the flapper to full heat manually, as you found the condition of yours initally.

The cable would only let it close so much, it SEEMED that it was fully closed, but after disconnecting the cable, I was able to get the smallest amount more movement out of the blend door when it was free of the cable.

For the coldest part of winter, I run with the cable disconnected and regulate the cab temp via the kick panel vents or the windows. Once we get into the transition seasons, I re-connect the cable, and can regulate the temp using the lever on the control module in the dash.

My first shot at repairing, a couple seasons ago, I changed out the heater core for a new one. Still the same semi-warm air entering the cab. Figured I'd replace the cable, and that didn't solve the issue either. Did verify that there was nothing prohibiting blend door movement within the heater core box.

I've accepted that it's just a quirk of the truck. Can't really see what other parts could be causing this, and found out the hard way that the loop end of the cable is not really flexible enough to adjust it on your own. It would rather break than bend.

[2badgn]

Heat control valve?